macOS Signing Scripts: Create the output directory if it does not exist.

Bug: 958163
Change-Id: I6e990717c0559d5ff4a087940a15470c72cc9e4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1606422Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658725}

chrome/installer/mac/sign_chrome.py

@@ -86,6 +86,9 @@ def main():
     config = create_config(args.identity, args.keychain, args.development)
     paths = model.Paths(args.input, args.output, None)
 
+    if not os.path.exists(paths.output):
+        os.mkdir(paths.output)
+
     pipeline.sign_all(paths, config, package_dmg=not args.no_dmg)